1. Field of the Invention
The present invention relates to timing systems, and, more particularly, to methods and apparatuses for measuring phase differences between signals and adjusting interval counters based on the measured phase differences.
2. Description of the Related Art
Timing and digital communication systems routinely use internal clocks to generate reference signals. Those systems use the reference signals to keep time and to generate other signals and codes used to communicate with other devices. In such systems there is a need to know the phases of the internal clocks to synchronize them with other clocks in transmitters and receivers in the communications system.
One timing system that uses frequency standards as timing sources is the Global Positioning System (GPS). The GPS system is a satellite-based spread-spectrum communications system that transmits coded signals from the satellites for use by receivers to determine their position and the time. The GPS satellites use redundant atomic frequency standards (AFS), i.e., atomic clocks, on each satellite as the basis for accurate timing with long-term stability. The atomic frequency standards include Cesium beam frequency standards, or Rubidium based frequency standards. In the GPS satellites the AFS signal is a very accurate signal with a frequency of nearly 13.4 MHz. However, the AFS frequency is determined by the physical attributes of the Cesium or Rubidium atoms, and is not precisely related by any simple ratio of integers to common time-keeping, which is based on the rotation of the earth. Furthermore, the atomic frequency standards are not easily tuned (adjusted).
Each GPS satellite also uses a less stable, but adjustable frequency source, namely, a voltage-controlled crystal oscillator (VCXO) to generate a 10.23 MHz xe2x80x9csystem clockxe2x80x9d which is used to generate timing signals used in the satellite to control the timing of navigation signals broadcast from the satellites. Although the system clock is not sufficiently stable by itself, it is adjustable; and by continually adjusting it using information obtained by comparison with the AFS frequency, the adjusted system clock can obtain the stability of the AFS. By comparing the 10.23 MHz system clock with the very accurate 13.4 MHz AFS clock signal, errors in the system clock can be determined and adjusted. Each GPS satellite uses a phase meter to compare these two clocks and to adjust the system clock. The phase meter data can also be used to monitor the AFS performance, to adjust the satellite timing to follow a world-wide time standard, and/or to create an ensemble clock, that is, to average the timing of multiple atomic clocks from one or more satellites, thus obtaining a virtual clock that is better than any one atomic clock alone.
In many applications, such as in GPS, the phase of a signal and its phase change must be measured with a high degree of precision because of the need to generate the highly stable frequency signals. In some applications where transmitters and receivers are widely distributed and those devices must remain closely synchronized for communications or other purposes, phase meters can be used to help maintain that synchronization. However, in many instances the precision of conventional phase meters is inadequate, thereby inhibiting the development of such systems. In other cases, high precision phase meters are too expensive for certain applications, or the technology used to build conventional high precision phase meters is incompatible with more economical technologies, thereby hindering large scale integration (LSI) of the phase meter.
Conventional methods for detecting a phase difference between two frequency signals, such as the 10.23 MHz GPS system clock and the 13.4 MHz AFS clock, use another clock signal that is very fast with respect to both of the other two frequency signals. Time is measured by counting the cycles of the very fast clock. That fast clock, however, must be as fast as possible, and thus becomes very expensive to achieve even modest precision. A problem with using the fast clock is that the logic technology enabling the clock to operate so fast is expensive making it infeasible to combine that fast logic with more economical logic technology used in large scale integration (LSI). That fast logic technology also consumes more power than does slower, more conventional logic technologies. An effect of the increased power consumption is that the size and weight of ancillary components such as power supplies and drivers must be increased. As a result, the fast logic required by conventional phase meters inhibits the integration of those phase meters with other less expensive logic technology. It also makes it infeasible to include additional phase meters in the satellite for measuring the phase of other signals such as the output of a back-up atomic clock.
Accordingly, there is a need to measure two or more clock frequency signals precisely and economically without requiring use of a faster clock signal in the measurement.
Therefore, in light of the above, and for other reasons that become apparent when the invention is fully described, an object of the present invention is to measure a phase difference between two clock signals of very different frequencies with a high degree of precision without requiring a separate fast clock signal.
A further object of the present invention is to determine the phase difference between two clock signals using economical logic technologies.
Yet a further object of the present invention is to detect the phase difference between two clock signals using a circuit implemented on an integrated circuit.
A still further object of the present invention is to measure the phase difference between two clock frequencies that are not very different from one another.
Another object of the present invention is to adjust an interval counter counting cycles of one of the clocks to be measured.
Yet another object of the present invention is to make a phase meter light-weight and that consumes little power.
The aforesaid objects are achieved individually and in combination, and it is not intended that the present invention be construed as requiring two or more of the objects to be combined unless expressly required by the claims attached hereto.
In accordance with the present invention, a phase meter measures the phase of a first signal with respect to a second signal. The phase meter includes a sampler that samples the first signal based on the second signal and outputs samples. It also includes a permuter that is coupled to the sampler, and permutes the samples based on the frequencies of the first and second signals. The permuter permutes the samples according to the formula j=A*i modulo C1, where C2/C1 approximates F1/F2, C1 and C2 are integers, F1 is a frequency of the first signal, F2 is a frequency of the second signal, and A=C2 modulo C1. The phase meter also includes a filter coupled to the permuter, that filters the permuted samples and outputs a phase measurement signal indicating the phase of the first signal.
In accordance with another embodiment of the invention, the phase meter can include an interval counter that determines the number of cycles of the first signal within an interval. It determines the number of cycles based on a number of transitions in the first signal determined from the samples. A count adjuster that is coupled to the interval counter, the sampler and matched filter, adjusts the number of cycles determined by the interval counter, based on the phase measurement signal and the samples. The count adjuster outputs an interval measurement signal that precisely reflects the full and part cycles within the interval, and therefore precisely measures the interval.
In accordance with yet another embodiment of the invention there is a method of measuring the phase of a first signal with respect to a second signal, where the two signals have very different frequencies. The method includes sampling the first signal with the second signal; permuting the samples based on the frequencies of the signals; filtering the permuted samples and outputting a phase measurement signal based on the filtered permuted samples.
The above and still further objects, features and advantages of the present invention will become apparent upon consideration of the following descriptions and descriptive figures of specific embodiments thereof. While these descriptions go into specific details of the invention, it should be understood that variations may and do exist and would be apparent to those skilled in the art based on the descriptions herein.